Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 48

The zinc blende structure has ccp/fcc anions with cations in one set of tetrahedral sites, either T₊ or T_–. The ZnS₄ tetrahedra are linked at their corners and each corner is common to four such tetrahedra. The unit cell of zinc blende, Fig. 1.29(b), is shown again in Fig. 1.33(a), but in terms of corner‐sharing ZnS₄ tetrahedra. The faces of the tetrahedra are parallel to the cp anion layers, i.e. the {111} planes; this is emphasised in a more extensive model in Fig. 1.33(b), which is oriented so that one set of tetrahedron faces is approximately horizontal. Conventionally, ZnS is regarded as built of cp layers of sulphides anions with the smaller zinc cations in tetrahedral sites. Since the same structure is generated by interchanging the Zn and S, the structure could also be described as a cp array of Zn with S occupying one set of tetrahedral sites. A third, equivalent description is as an array of ccp ZnS₄ (or SZn₄) tetrahedra.

Some compounds with the zinc blende structure are listed in Table 1.9. The bonding is less ionic than in compounds with the rock salt structure. Thus, oxides do not usually have the zinc blende structure (ZnO, not included in Table 1.9, is an exception; it is dimorphic with zinc blende and wurtzite structure polymorphs). Chalcogenides of the alkaline earth metals (but not Be) have the rock salt structure, whereas the chalcogenides of Be, Zn, Cd, and Hg have the zinc blende structure, as do Cu(I) halides and Γ ‐AgI. A range of III–V compounds (i.e. elements from Groups III and V of the Periodic Table) have the zinc blende structure and some, e.g. GaAs, are important semiconductors.

Table 1.8 Some compounds with the NaCl structure, a/Å

MgO	4.213	MgS	5.200	LiF	4.0270	KF	5.347
CaO	4.8105	CaS	5.6948	LiCl	5.1396	KCl	6.2931
SrO	5.160	SrS	6.020	LiBr	5.5013	KBr	6.5966
BaO	5.539	BaS	6.386	LiI	6.00	KI	7.0655
TiO	4.177	α‐MnS	5.224	LiH	4.083	RbF	5.6516
MnO	4.445	MgSe	5.462	NaF	4.64	RbCl	6.5810
FeO	4.307	CaSe	5.924	NaCl	5.6402	RbBr	6.889
CoO	4.260	SrSe	6.246	NaBr	5.9772	RbI	7.342
NiO	4.1769	BaSe	6.600	NaI	6.473	AgF	4.92
CdO	4.6953	CaTe	6.356	TiN	4.240	AgCl	5.549
TiC	4.3285	LaN	5.30	UN	4.890	AgBr	5.7745

Figure 1.33 The sphalerite (zinc blende) structure showing (a) the unit cell contents and (b) a more extended network of corner‐sharing tetrahedra.